The majority of all laminate floors are produced according to a production method generally referred to as Direct Pressed Laminated (DPL). Such laminated floors comprise a core of a 6-12 mm fibre board, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate, plastic, paper or like material.
The surface layer of a laminate floor is characterized in that the decorative and wear properties are generally obtained with two separate layers of paper, one above the other. The decorative layer is generally a printed paper and the wear layer is a transparent overlay paper, which comprises small aluminium oxide particles.
The printed decorative paper and the overlay are impregnated with melamine formaldehyde resins and laminated to a HDF core in large discontinues or continuous laminate presses where the resin cures under high heat and pressure and the papers are laminated to the core material. An embossed press plate or steal belt forms the surface structure. Sometimes a structured paper is used as a press matrix.
Laminated floors may also be produced with printing technology. One advantage is that the pressing operation may be avoided and that no printed papers are needed to provide a decorative wear resistance surface.
Floor panels with a Direct Printed Laminate surface comprise the same type of HDF core as DPL. The décor is printed directly onto the core. The production process is rather complicated and is only cost efficient in very large production volumes. Hydro printing inks are used to print the décor by a multicolour printing press with rollers that print directly onto the pre-sealed core.
Direct printing technology may be replaced with Digital Printing Technology that is much more flexible and small production volumes can be economically manufactured. The difference between these two methods is mainly the printing step where printing rollers are replaced by a digital non-contact printing process and where the desired image is directly applied on to the pre-finished core.
Digital printing may also be used to print on a paper sheet that is used in conventional laminate production and laminated under heat and pressure. The printing may be made prior to or after impregnation.
Paper and plastic foils are also used as surface layers in flooring and such materials may also be printed digitally.
Recently new “paper free” floor types have been developed with solid surfaces comprising a substantially homogenous powder mix of fibres, binders and wear resistant particles.
The powder mix may comprise aluminium oxide particles, melamine formaldehyde resins and wood fibres. In most applications decorative particles such as, for example, colour pigments are included in the mix. In general all these materials are applied in dry form as a mixed powder on a HDF core and cured under heat and pressure to a 0.1-1.0 mm solid layer. The powder is prior to pressing stabilized with moisture and UV lamps such that it forms an upper skin layer similar to a paper layer and this prevents the powder from blowing away during pressing. Melamine formaldehyde resin and wood fibres may be replaced by thermoplastic particles.
Several advantages over known technology and especially over conventional laminate floorings may be obtained such as increased wear and impact resistance, deep embossing, increased production flexibility and lower costs.
Powder technology is very suitable to produce a decorative surface layer, which is a copy of stone and ceramics. It is however more difficult to create designs such as, for example, wood decors. However, recently digital powder printing has been developed and it is possible to create very advanced designs of any type by injecting ink into the powder and create a digital print in the powder prior to pressing. The surface structure is made in the same way as for laminate flooring by a structured press plate, steal belt or an embossed matrix paper that is pressed against the powder.
Floors with a surface of wood are produced in many different ways. Traditional solid wood floors have developed into engineered floors with wood layers applied on a core made of wood lamellas, HDF or plywood. The majority of such floors are delivered as pre-finished floors with a wood surface that is coated with several transparent layers in the factory. Recently wood floorings have also been produced with a digitally printed pattern that improves the design of the wood grain structure in wood species that do not have a sufficient surface quality.
Digital printing is used in several floor types to create a décor. However the volumes are still very small mainly due to the high cost of the ink and the high investment cost for the industrial printers. It would be a major advantage if the ink cost could be reduced and if more cost efficient equipment could be used in an industrial scale.
Definition of Some Terms
In the following text, the visible surface of the installed floor panel is called “front side”, while the opposite side of the floor panel, facing the sub floor, is called “rear side”. By “surface layer” are meant all layers, which give the panel its decorative properties and its wear resistance.
By “print” is meant a décor or image. By “up” is meant towards the front side and by “down” towards the rear side. By “vertically” is meant perpendicular to the surface and by “horizontally” parallel to the surface.
By “pigments” is meant a very fine powder of solid colorant particles.
By “Pigment ink” is meant an ink comprising pigments that are suspended or dispersed throughout a carrier fluid.
By “dye ink” is meant a coloured substance that is dissolved fully into the carrier fluid and the resultant ink is a true solution completely soluble like sugar in water.
By “aqueous or water based ink” is meant an ink where water is used as liquid substance in the ink. The water-based liquid carries the pigments.
By “solvent based ink” is meant ink that generally contains three major parts such as a fluid carrier, pigments and resins. Technically, solvent ink refers generally only to the oil-based carrier portion of the ink that keeps the other components in liquid form and once applied to a surface through jetting evaporates.
By “UV curable inks or coating” is meant ink or coating that after application is cured by exposure to strong UV-light in an UV oven.
By “binder” is meant a substance that connects or contributes to connect two particles or materials. A binder may be liquid, powder based, a thermosetting or thermoplastic resin and similar.
Known Technique and Problems Thereof
The general technology, which is used by the industry to provide a digital print, is described below. The methods described below may be used separately or in combinations to create a digital print or a digital application of a substance in the embodiments of this disclosure.
High definition digital printers use a non-impact printing processes. The printer has print heads that “fire” drops of ink from the print heads to the substrate in a very precise manner.
Multipass printing, also called scanning printing, is a printing method where the printer head moves transverse above the substrate many time to generate an image. Such printers are slow but one small print head can generate a bigger image.
Industrial printers are generally based on a Single Pass printing method, which uses fixed printer heads, with a width that corresponds to the width of the printed media. The printed substrate moves under the heads. Such printers have a high capacity and they are equipped with fixed print heads that are aliened one after each other in the feeding direction. Each print head prints one colour. Such printers may be custom made for each application.
FIG. 1a shows a single pass printer 35 comprising five digital print heads 30a-e, which are connected with ink pipes 32 to ink containers 31 that are filled with ink of different colours. The print heads are connected with digital data cables 33 to a digital control unit 34 that controls the application of the ink drops and the speed of the conveyor 21 that displaces the panel under the print heads with high precision in order to guarantee a high quality image comprising several colours. FIG. 1b shows a wood grain print P provided on a panel surface 2. The surface of a floor panel is often embossed with a standard structure 17 that is the same for several basic decors as shown in FIG. 1c. Advanced floors use an embossing 17 that is in register with the printed pattern P as shown in FIG. 1d. 
A normal width of an industrial print head is about 6 cm and any lengths may be printed. Wide areas of 1-2 m may be printed with digital printers comprising several rows of print heads aligned side by side.
Number of dots per inch or DPI is used to define the resolution and the printing quality of a digital printer. 300 DPI is generally sufficient to, for example, print wood grains structures of the same quality presently used in conventional laminate floorings. Industrial printers can print patterns with a resolution of 300-600 DPI and even more and with a speed exceeding 60 m/min.
The print may be a “full print.” This means that the visible printed décor is mainly created by the ink pixels applied on the surface. The colour of a powder layer or a base colour of a paper has, in such an embodiment, in general a limited effect on the visible pattern or decor.
The print may also be a “part print”. The colour of another underlying layer is one of the colours that are visual in the final décor. The area covered by printed pixels and the amount of ink that is used may be reduced and cost savings may be obtained due to lower use of ink and increased printing capacity compared to a full print design. However a part print is not as flexible as a full print since the base colours are more difficult to change than when a full print is used.
The print may be based on the CMYK colour principle. This is a 4-colour setup comprising cyan, magenta, yellow and black. Mixing these together will give a colour space/gamut, which is relatively small. To increase specific colour or the total gamut spot colours may be added. A spot colour may be any colour. The colours are mixed and controlled by a combination of software and hardware (print engine/print heads).
New technology has been developed by Välinge Innovation AB that makes it possible to inject a digital print into a powder layer. This new type of “Digital Injection Print” or DIP is obtained due to the fact that printing is made into a powder that is cured after printing. The print is embedded into the cured layer and is not applied on a layer as when conventional printing methods are used. The print may be positioned in several dimensions horizontally and vertically in different depths. This may be used to create 3D effects when transparent fibres are used and to increase the wear resistance. No protective layers are needed that disturb the original design.
The DIP method may be used in all powder based materials, which may be cured after printing. However, the DIP method is especially suitable to be used when the powder comprises a mix of wood fibres, small hard wear resistant particles and a melamine formaldehyde resin. The surface layer may also comprise thermoplastic material, for example, vinyl particles, which are applied in powder form on a substrate. This allows that the print may be injected in the vinyl powder particles. An improved design and increased wear resistance may be reached even in such materials.
A suitable printer head has to be used in order to obtain a high printing quality and speed in powder based layers and other layers as described above. A printer head has several small nozzles that can shoot droplets of inks in a controlled way (Drop On Demand-DOD). The size of each droplet may vary, dependant on ink type and head type, between normally 1-100 picoliters. It is possible to design print heads that may fire bigger drops up to 200 picoliters more. Some printer heads can shoot different droplet sizes and they are able to print a greyscale. Other heads can only shoot one fixed droplet size.
Different technologies may be used to shoot the drops out of the nozzle.
Thermal printer head technology use print cartridges with a series of tiny chambers each containing a heater, all of which are constructed by photolithography. To eject a droplet from each chamber, a pulse of current is passed through the heating element causing a rapid vaporisation of the ink in the chamber to form a bubble, which causes a large pressure increase, propelling a droplet of ink out through the nozzle to the substrate. Most consumer inkjet printers, from companies including Canon, Hewlett-Packard, and Lexmark use thermal printer heads.
Most commercial and industrial inkjet printer heads and some consumer printers such as those produced by Epson, use the piezoelectric printer head technology. A piezoelectric material in an ink-filled chamber behind each nozzle is used instead of a heating element. When a voltage is applied, the piezoelectric material changes shape, which generates a pressure pulse in the fluid forcing a droplet of ink from the nozzle. Piezoelectric (also called Piezo) inkjet allows a wider variety of inks than thermal inkjet, as there is no requirement for a volatile component, and no issue with kogation. A lot of ink types may be used such as dye inks, solvent based inks, latex inks or UV curable inks.
Pigment based inks are generally individually mixed together by using colour pigments and several chemicals. A pigment is a very fine powder of solid colorant particles that are suspended or dispersed throughout a liquid carrier. Pigments used in digital ink have an average particle size of about 0.1 micron. The common size of the nozzles are about 20 microns which meant that the pigment particle have enough space to pass through the nozzle channels in the print head. The nozzles may still be blocked by the ink itself and pigments that form clusters of particles. A high quality pigment ink should keep the pigment suspended in the carrier fluid for a long period of time. This is difficult particularly at the low viscosities that are required for a good functioning of the print heads. Pigments have a natural tendency to settle out and fall down in the liquid carrier. In high quality pigment ink, no settling out of the pigment should normally occur.
Water based inks comprising colour pigments are especially suitable and may provide a high quality printing method in many different materials. Pigment inks are generally more light fast and more fade resistant than dye-based inks.
The pigments do not stick to a surface. They are similar to sand particles and may be easily removed from most dry surfaces. The water based carrier fluid is therefore generally mixed with small amounts of several other additives to provide special ink and print properties, such as binders that provide the adhesion of the pigments to a surface, dot gain, pH level, drop formation, corrosion of the print head, fade resistance etc.
Colour pigments as such are rather cost competitive but the production of pigment based inks and other inks for digital printers is very complicated and expensive and this results in a very high cost for the ink that normally may be in the region of about 100 EUR/liter. About 100 m2 of flooring may be printed with one liter if a full high quality print is applied and this gives a cost of 1 EUR/m2. The costs for a conventional printed floor surfaces where printing cylinders are used are only 10% of the cost for digitally printed floor surfaces.
Digital ink jet printers use a non-contact method to apply the ink on a surface. Laser printing however is based on a contact method where a laser beam projects an image on an electrically charged rotating drum. Dry ink particles, generally called toner, are then electrostatically picked up by the drum's charged areas. The ink comprises fine particles of dry plastic powder mixed with carbon black or colouring agents. The thermosetting plastic material acts as a binder. The drum prints the image on a paper by direct contact and heat, which fuses the ink to the paper by bonding the plastic powder to the paper. Colour laser printers use the CMYK principle with coloured dry ink, typically cyan, magenta, yellow, and black that are mixed in order to provide a high quality coloured image.
The laser technology with the impact method is not used for printing of a flat panel surfaces such as a floor panel surfaces.
The above description of various known aspects is the applicants' characterization of such, and is not an admission that any of the above description is prior art. Several of the technologies described above are known and used individually but not in all combinations and ways as described above.
As summary it may be mentioned that digital printing is a very flexible method but it cannot be fully utilized due to the high cost for the ink. The costs are primarily caused by the need to mill down the colour pigments to well-defined very small particles and to disperse the particles throughout the carrier fluid. It would be a major advantage if digital images may be created with ink that does not contain colour pigments or colour substances.
The digital application technology is only used to obtain advantages related to the possibility to create a high-resolution image in a flexible way. However, the other aspects of the technology, mainly related to the possibility to apply a liquid substance very precisely with a non-impact method, have not been fully utilized or developed.
It is known that powder applied on a liquid substance could be used to create raised portions or an image on mainly a paper substrate and that the liquid substance may be applied digitally by ink jet.
U.S. Pat. No. 3,083,116 describes raised printing powder and a raised printing process comprising dusting a powdered resin upon a newly printed sheet, removing therefrom the excess powder which do not adhere to the wet ink, and applying heat to the powder retained on the sheet to fuse it so that particles thereof will flow together and adhere to the sheet. The powder may comprise a phenolic resin.
U.S. Pat. No. 3,446,184 describes a method to form a sticky image copy. Toner powder is applied on a liquid forming and a portion of the powder is retained by the liquid coating, forming a visible image. Loose powder is removed and the sheet passes a heating unit where the retained powder is fused to form a permanent image.
U.S. Pat. No. 4,312,268 describes a method by which a water-based ink is applied digitally to a continuous web and fusible single colour powder material is applied to the web and on the ink. Some of the powder material is bonded to the liquid, and non-bonded powder material is removed from the web prior to heating of the web to dry the liquid and to fuse the powder material to the web by melting the powder. It is mentioned that the powder material may have a particle size in the range of 5 to 1000 microns and may have a melting point or fusing point in the range of 50 to 300 degrees Centigrade. The powder material may be produced by dissolving or dispersing, respectively, a dye or a pigment in a resin or resin formulation, followed by grinding, spray chilling or the like to reduce the material to a fine powder. The powder material may provide abrasion resistant qualities to the ink that may contain phenolic resin. The liquid material, which is applied through the jets, may be clear and colourless water.
U.S. Pat. No. 6,387,457 describes a method of printing using dry pigments. A binder material is applied to a surface of a substrate uniformly or in a pattern. Dry pigment is applied to the binder material in a pattern or uniformly. The dry pigment material comprises flakes of non-metallic material having a particle size less than about 100 micron. The flakes are aligned in a direction parallel with the surface of the substrate.
EP 0 403 264 A2 describes a transfer method to form a multi-colour image on a drum that transfers the image to a paper. A fluid digital latent image is subsequently developed at a development station where coloured powder is applied to the fluent latent image and fixed to produce a visible and permanent image. Several digital print heads may be used that print with dyeless fluids comprising a mixture of water with polyhydric alcohols and their sub-sets of ethylene glycol, glycerol, diethylene glycol and polyethylene glycol. A powder toner is applied across the surface of the paper and a voltage is applied during this development. The voltage is then reversed to remove the toner from the background areas. Fixing is achieved by means of conventional copier fusing methods.
EP 0 657 309 A1 describes a multicolour transfer method utilizing a transfer paper carrying a pattern formed by ink jet and powder similar to the above described methods. The transfer method is intended for decorating ceramics.
WO 2011/107610 describes a method to create an elevation or an embossing on a floor panel in order to avoid the use of expensive press plates. The method is the same as the known methods to create a raised print. It describes a method to produce a floorboard by printing a curable substance for creating an elevation on the panel. The elevation may be applied on a basic decorative pattern that is directly printed or laminated on the panel. The curable substance may comprise wear resistant particles. The curable substance may be digitally printed on the panel by first printing a liquid in a pre-defined pattern and then providing an intermediate substance that may comprise a powder. The curable substance may be cured by UV radiation or may be a varnish.
The known methods are not suitable for creating a high quality multi-colour image on a building panel, and especially not on a floor panel where UV resistant pigments must be used and where the image must be incorporated into a wear resistant surface. It is not known that the known principles may be used to create an image on a flooring surface that is pressed and especially not how the principles should be adapted for printing of floor surfaces similar to laminate and Wood Fibre Floors (WFF) where the powder, the ink and the application methods must be adapted to the specific resins, materials and pressing parameters which are needed to form a wear, impact and stain resistant high quality multi-colour surface in a cost efficient way.